Attenuation systems and methods for use with an optical detector in an optical communication system

ABSTRACT

Systems and methods for use with an optical communication beam are disclosed. The system allows the beam of light to operate at an adequate power level that provides a robust optical link while minimizing any safety risk to humans. The system calibrates and controls the gain for an avalanche photodiode detector (APD). A detector circuit is used to calibrate the APD. Once calibrated, the detector circuit further provides an electrical bias to the APD to process or condition the electrical signal to produce a detector output. The systems and methods disclosed herein attenuate the power level of an incoming communication beam to prevent oversaturation of an APD. The system further provides an alignment signal, which is effective over a wide dynamic range of incoming power levels.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/513,874, entitled “ATTENUATION AND CALIBRATION SYSTEMS AND METHODSFOR USE WITH A LASER DETECTOR IN AN OPTICAL COMMUNICATION SYSTEM”, filedAug. 31, 2006, hereby incorporated by reference, which is a continuationapplication of U.S. patent application Ser. No. 10/045,661, entitled“ATTENUATION AND CALIBRATION SYSTEMS AND METHODS FOR USE WITH A LASERDETECTOR IN AN OPTICAL COMMUNICATION SYSTEM”, filed Oct. 23, 2001,hereby incorporated by reference, which is a continuation-in-partapplication of U.S. patent application Ser. No. 09/941,319, filed Aug.28, 2001, titled “Automatic Laser Power Control in an OpticalCommunication System” which claims priority to U.S. provisional patentapplication Ser. No. 60/240,346, filed Oct. 13, 2000, both priorityapplications are hereby incorporated by reference. This application alsoclaims priority to U.S. provisional patent application Ser. No.60/242,539, filed Oct. 23, 2000, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to systems and methods for free-spaceoptical communication networks and to a system and method forcontrolling the power of a laser used in such a network.

2. Description of the Related Art

Currently, the primary method for data transmission between remotelocations utilizes wired lines or fiber optic cables. Some of the costsassociated with this method are due to the expense in obtainingrights-of-way for the cable runs as well as installing the cables byburying or hanging. While this method has proven successful where greatdistances separate two locations, it is prohibitively expensive betweenlocations that are within close proximity to one another.

The dramatic growth in the demand for broadband services and the timeand expense associated with deploying traditional wired lines or fiberoptic cables have led to the development of new wireless broadbandaccess technologies. One of these new wireless technologies employs aLight Amplification Stimulated Emission of Radiation (laser) beam totransmit information. Such a system may consist of at least 2 opticaltransceivers accurately aligned to each other with a clear line-of-sightto deliver the information using such a laser beam.

However, when the communication laser beams are present in a locationaccessible by people, laser safety becomes an important issue. Unlikelight produced by a common lamp or the sun, laser light is not divergentand often emits radiation within a narrow band of wavelengths to form amonochromatic light. Furthermore, because this laser light is coherentand non-divergent, it is easily focused by the lens of a human eye toproduce images on the retina with greater intensity than is possiblewith these other common sources of light.

Safety guidelines do exist for the use of lasers. For example, suchguidelines are promulgated by the International ElectrotechnicalCommission (IEC) based on a maximum permissible exposure (MPE) level. Ifone were to apply such a standard, a maximum power level could bepredicted (known as an Accessible Emission Limit (AEL)) that would makethe communication laser beam eye-safe to a viewer, known as a class 1laser system in the IEC standard. However, to establish and maintain ahigh-bandwidth connection, the lasers used in such systems may transmitat power levels that exceed the class 1-power levels designated by theselaser safety guidelines.

Therefore, there is a need for a system and a method that allows the useof optical communication beams of light with adequate power to provide arobust optical link between communication terminals while minimizingsafety risks to either users or a passerby. Such a system and method maymaintain a signal-to-noise ratio above a desired value at the distantreceiving communication terminal and under various environmentalconditions that tend to degrade the signal, such as fog, smog, rain, orsnow. Moreover, such a system and method could expand the permissiblelocations for placement of such optical transceivers to places that areaccessible to humans.

Optical-to-electronic conversion of a communication laser beam is animportant process. In many optical communication systems, for example,an information-bearing optical wave, after transmission through anoptical link, is received by an optical detector within the transceiver.The optical detector converts the optical wave into an electrical signalfor further processing. The optical detector can include a photosensorand a detector circuit coupled thereto. The photosensor, such as aphotodiode, converts the received photons of the optical wave into anelectrical signal. This electrical signal is in the form of photocurrent or a photo voltage. For a given photosensor, the design andoperation of its detector circuit can be configured to enhance theadvantages and suppress disadvantages of the photosensor for a specificapplication. For example, the detector circuit can be used to calibratethe photosensor. Once calibrated, the detector circuit can furtherprovide an electrical bias to the photosensor to process or conditionthe electrical signal to produce a detector output.

Laser transmitter and detector sensitivity dynamic ranges are oftenmismatched. Laser transmit power control typically have a more limiteddynamic range that the laser detector. When optimal weather conditionsoccur between a laser transmitter and a detector, the laser transmittercan oversaturate the detector. Due to the limited dynamic range of thelaser transmitter, the system may be unable to reduce the lasertransmitter's power to prevent oversaturation. Additionally, incidentlight is reflected by the receiver and in a direction towards thetransmitting laser. A receiver, which is associated with thetransmitting laser, may experience interference with its incomingcommunication beam from this reflected light.

The quality of a received signal is often degraded when a detector isnot aligned with the incoming communication beam. Alignment between atransmitter and a receiver is often performed using a signal transmittedbetween the transmitter and receiver. However, the signal's power mayhave a wide dynamic range which is difficult to process by the receiver.

Thus there is a need for system and method which calibrates aphotosensor and enhances its operational dynamic range. The systemshould also attenuate the power level of an incoming communication beamto prevent oversaturation of a receiver. The system should furtherprovide an alignment signal which is effective over a wide dynamic rangeof incoming power levels.

SUMMARY OF THE INVENTION

The systems and methods have several features, no single one of which issolely responsible for its desirable attributes. Without limiting thescope as expressed by the claims which follow, its more prominentfeatures will now be discussed briefly. After considering thisdiscussion, and particularly after reading the section entitled“Detailed Description of the Preferred Embodiments” one will understandhow the features of the system and methods provide several advantagesover traditional communication systems.

One aspect is a system for calibrating an avalanche photodiode detector(APD) for use in an optical communication system which comprises acurrent sense module configured to measure a receive (Rx) power outputvalue for an APD, a high voltage control (HVC) configured to provide avariable voltage bias to the APD in accordance with a high voltagecontrol signal, and a thermal sensor configured to measure a temperatureof the APD. The system further includes a processor configured toprovide the high voltage control signal to the HVC, wherein the highvoltage control signal is based on the temperature and the Rx poweroutput value, and a high voltage supply configured to provide voltage tothe HVC.

Another aspect is a method for calibrating an avalanche photodiodedetector (APD) for use in an optical communication system whichcomprises turning off transmitted optical power incident on the APD tolimit light from reaching the APD, lowering a bias voltage for the APDto zero volts, and once lowered, measuring an initial conduction for theAPD. The method further includes storing the initial conduction,incrementally increasing the bias voltage until current is sensedthrough the APD, and once current is sensed, measuring a maximum biasvoltage across the APD. The method still further includes determining acalibration value based on the initial conduction and the maximum biasvoltage and applying the calibration value to the APD.

Another aspect is a system for increasing an operational dynamic rangeof an avalanche photodiode detector (APD) for use in an opticalcommunication system comprising a current sense module configured tomeasure an incoming photo current to an APD, a high voltage control(HVC) configured to reduce a variable voltage bias to the APD inresponse to a decrease in the incoming photo current whereby an APD gainvalue is simultaneously decreased, a processor configured to control thevariable voltage bias using a high voltage control signal based on theincoming photo current measured by the current sense module, and a highvoltage supply configured to provide voltage to the HVC.

Another aspect is a method for increasing an operational dynamic rangeof a variable gain avalanche photodiode detector (APD) for use in anoptical communication system comprising setting a voltage bias for anAPD, sensing a reduction in incoming photo current to the APD, andreducing the voltage bias of the APD such that a gain value applied tothe photo current is reduced, wherein an operational dynamic range ofthe APD is increased.

Another aspect is a method for controlling incoming laser power in acommunication system which includes a first node and a second node wherethe second node transmits a first communication beam to the first nodeand where the first node includes a first optical attenuator. The methodcomprises monitoring the receive (Rx) power level of a photodiodedetector in a first node, determining if the Rx power level exceeds asaturation threshold level for the photodiode detector, if the Rx powerlevel exceeds the saturation threshold level of the photodiode detector,enabling a first optical attenuator that is located in a path betweenthe first communication beam and the photodiode detector, and if the Rxpower level is below a minimum threshold level of the photodiodedetector, disabling the first optical attenuator.

Another aspect is a system configured for controlling incoming laserpower in a communication system which includes a first node and a secondnode where the second node transmits a communication beam to the firstnode. The system comprises a first node having a photodiode detectorconfigured to receive an incoming communication beam, a first opticalattenuator coupled to the first node and configured to attenuate theincoming communication beam prior to it reaching the photodiodedetector, a second node configured to transmit the incomingcommunication beam, and a first attenuation control module configured tocontrol the first optical attenuator to maintain a power level of theincoming communication beam to within an operational range of thephotodiode detector.

Another aspect is a system for aligning an optical receiver to anincoming communication beam for use in an optical communication systemcomprising an avalanche photodiode detector (APD) configured to converta communication beam into a photo current, an amplifier configured toconvert the photo current into a voltage signal, and a processingcircuit configured to convert the voltage signal into a received signalstrength indicator (RSSI). The system further comprises a current sensemodule configured to measure a receive (Rx) power signal for the APD, anactuator configured to align the APD with the communication beam, and aprocessor configured to control the actuator based on a combined powersignal which includes the RSSI and the Rx power signal.

Another aspect is a method for aligning an optical receiver to anincoming communication beam for use in an optical communication system,wherein the optical communication system includes a first node and asecond node, each including a movable avalanche photodiode detector(APD) configured to receive a communication beam from the other node.The method comprises converting an incoming communication beam to an APDinto a photo current, converting the photo current into a voltagesignal, and determining a received signal strength indicator (RSSI) fromthe voltage signal. The method further comprises determining a receive(Rx) power signal for the APD, and aligning the APD with thecommunication beam based on the RSSI and the Rx power signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements.

FIG. 1 is a diagram illustrating an example communication network.

FIG. 2 is a diagram illustrating an example implementation of a node.

FIG. 3 is a block diagram illustrating a blocked communication linkbetween two node heads of two nodes.

FIG. 4 is a graph of the power levels and associated durations of aninterrupted beam of radiation.

FIG. 5 is a graph of the power levels and associated durations of aninterrupted beam of radiation.

FIG. 6 is a block diagram of a control module from FIG. 3.

FIG. 7 is a flow chart illustrating a power reduction process performedby the control module.

FIG. 8 is a flow chart of an acquisition and recovery process performedby the control module.

FIG. 9 is a block diagram of a control module from FIG. 3 configured tooptimize the characteristics of an Avalanche Photodiode Detector (APD).

FIG. 10 is a block diagram of a receiver from FIG. 9 showing the APD andthe components related thereto.

FIG. 11 is a schematic diagram of a current sense module from FIG. 10for use during a calibration process.

FIG. 12 is a flow chart illustrating a calibration process for the APDthat is performed by the control module.

FIG. 13 is a schematic diagram of a high voltage control (HVC) modulefrom FIG. 10 which operates in conjunction with a resistor from thecurrent sense module to enhance the operational dynamic range of theAPD.

FIG. 14 is a graph of APD gain versus APD voltage bias.

FIG. 15 is a block diagram of transceivers 308(a), 308(b) from FIG. 3,showing a reflected signal from receiver 306(b) interfering withcommunication beam 110(b) at receiver 306(a).

FIG. 16 is a graph showing the transmission percent versus wavelengthfor the electrochromatic window when the electrochromatic window isactivated and when the electrochromatic window is deactivated.

FIG. 17 is a graph of APD operating range versus time, showing theeffect of activating the electrochromatic window to reduce the photocurrent to the APD from a communication beam.

FIG. 18 is a graph of APD operating range versus time, showing theeffect of deactivating the electrochromatic window to increase the photocurrent to the APD from a communication beam.

FIG. 19 is a flow chart illustrating an attenuation process foradjusting the power of the incoming communication beam into the APD thatis performed by the control module.

FIG. 20 is a block diagram of a receiver from FIG. 3 configured to allowan attenuating window to be removed from the path of a communicationbeam.

FIG. 21( a) is a plan view of the attenuating window from FIG. 20,further configured to incrementally attenuate a communication beam usingsectors of the attenuating window.

FIG. 21( b) is a plan view of the attenuating window from FIG. 21( a)showing non-adjacent sectors in a colored state.

FIG. 21( c) is a plan view of the attenuating window from FIG. 20,further configured to incrementally attenuate a communication beam usingareas between circles of the attenuating window.

FIG. 21( d) is a plan view of the attenuating window from FIG. 21( c)showing areas formed by circles 2404(a) and 2404(b) along withnon-adjacent area formed by circle 2404(c), both in a colored state.

FIG. 22 is a flow chart illustrating an incremental attenuation processfor adjusting the power of the incoming communication beam into the APDthat is performed by the control module, and includes removing theattenuating window from the path of the communication beam.

FIG. 23 is a block diagram of a receiver from FIG. 9 showing amisaligned, incoming, communication beam into the APD.

FIG. 24 is a graph of Rx power output versus incident angle for anincoming communication beam showing low sensitivity of the Rx poweroutput signal occurring at lower power levels.

FIG. 25 is a graph of receive signal strength indicator (RSSI) versusincident angle for an incoming communication beam showing clipping ofthe RSSI occurring at higher power levels.

FIG. 26 is a graph of the RSSI and the Rx power output signal combinedfor use with aligning an incoming communication beam into the APDirrespective of power level.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A free-space communication network may consist of at least two pairs ofoptical receivers and transmitters accurately aligned with each otherwith a clear line-of-sight to deliver high-bandwidth access over the airusing beams of optical radiation, commonly called light. The light'swavelength is a function of a selected laser medium. Such laser mediumsinclude, for example, solids, gases or liquids. The wavelengths form acontinuous range but are often broken into specific regions, forexample, infrared radiation (800 nanometer-10 microns), visible light(400 nm-700 nm), ultraviolet radiation (300 nm-3 nm), x-rays and gammarays (<3 nm). In one embodiment, the optical receiver and transmitterare combined into an optical transceiver. Each optical transceiver caninclude at least one Light Amplification Stimulated Emission ofRadiation (laser) and an optical detector. Embedded within the beams ofradiation from the transmitter is information, for example, in the formof data, voice, and video. The corresponding receiver, which has anoptical detector and associated signal processing circuit may convertthe information into an electrical signal for further routing orprocessing.

FIG. 1 is a diagram illustrating an exemplary communication network 100.The communication network 100 includes a plurality of nodes 108,interconnected by communication links 110. Each communication link 110includes two opposing beams of radiation between two nodes (i.e.incoming and outgoing beams). Certain of the communication links 110 maybe radio links or microwave links under appropriate circumstances.According to one embodiment, the nodes 108 are disposed on facilities104. Although only one node 108 is provided per facility in the exampleillustrated in FIG. 1, more than one node 108 can be provided at one ormore of facilities 104, depending on the communication requirements, andalso, perhaps, depending on the particular facility. Facilities 104 canbe buildings, towers, or other structures, premises, or locations.

Nodes 108 are interconnected with one another by optical communicationlinks 110. Nodes 108 include one or more optical transmitters andreceivers to provide the communication links 110 among the plurality ofnodes 108. The transmitters and receivers at nodes 108 can beimplemented using, for example, lasers or light emitting diodes (LEDs)as the optical transmitters and charge-coupled devices (CCDs),photomultiplier tubes (PMTs), photodiode detectors (PDDs) or otherphotodetectors as the receivers. Although the network 100 illustrated inFIG. 1 is illustrated as a mesh network structure, other networkstructures or geometries can be implemented. For example, in oneembodiment, branching tree architecture is used. In one embodiment, thenodes 108 include the capability to interface with up to four separatecommunication links 110.

Still referring to FIG. 1, network 100 provides a two-way connectionbetween one or more users in one or more facilities 104 and with aprovider network 116 via a root node 114. The root node 114 connectswith the provider network 116 via another communication link 112. In oneembodiment, the provider network 116 is a high bandwidth copper or fiberservice provider. Although only one provider network 116 is illustratedin FIG. 1, one or more root nodes 114 can be used to interface to morethan one provider network 116.

FIG. 2 is a diagram illustrating an example implementation of a node 108which is generally cylindrical in shape and can include four node heads200 and a node base 202. Node heads 200 each include a transceiver (notshown) to facilitate communication with one or more other nodes 108 in anetwork 100 (see FIG. 1). Each node head 200 provides a two-waycommunication link 110 with one other node head in the network 100 at agiven time. Thus, where each node head 200 has a single transceiver,node 108 communicates with up to four other nodes 108 at four separatelocations. Alternatively, two node heads can provide parallel links to asingle node. Other numbers of node heads 200 can be included, dependingon the fan-out capability desired for the node 108. Node 108 furtherincludes a drop 204 for connecting to a user. In one embodiment, thedrop is hardwired between the node base 202 and into a facility 104 (seeFIG. 1).

Node base 202 includes electronics and mechanics to provide acommunication interface between, for example, a provider network 116 andthe one or more node heads 200 via a communication link 112 (see FIG.1). A communications interface to perform protocol or format conversionscan be included in the node base 202 as well as mechanics to drive thepointing of one or more node heads 200.

One embodiment of the communication network 100 uses an opticaltransmission and multiplexing scheme for transferring data between thenodes 108 and the provider network 112. Such schemes use a physicallayer technology to handle the actual transmission and reception ofdata. In one embodiment, synchronous optical network (SONET) is usedwhich the American National Standards Institute standardizes. In anotherembodiment, synchronous digital hierarchy (SDH) is used which theInternational Telecommunications Union standardizes. The basic SONETchannel transmits 52 Mbps or OC-1. Higher transfer rates are obtainedwith the use of multiplexing. For example, a transfer rate of 155 Mbps,or OC-3, is achieved where three OC-1 channels are byte-interleaved.

FIG. 3 is a block diagram illustrating a blocked communication linkbetween two node heads 200(a), 200(b) of two nodes 108(a), 108(b). Node108(a) includes a node base 202(a) coupled to at least one node head200(a) via communication electronics 300. Node 108(b) includes a nodebase 202(b) coupled to at least one node 200(b) via communicationelectronics 300. Communication electronics 300 interface each node head200(a), 200(b) to node base 202(a), 202(b). In one embodiment, thecommunication electronics 300 includes a bus which connects the nodeheads 200(a), 200(b) to their respective node bases 202(a), 200(b). Inembodiments where each node 108(a), 108(b) includes multiple node heads,a multiplexer can be provided as part of the communication electronics300 to allow communications among the various elements over a sharedbus.

Each node head 200 can include a pointing mechanism such that it can berotated to point to a designated other node 108. Such pointing can beperformed in both azimuth and elevation. Ideally, each node head 200 canbe independently pointed to a designated node 108.

Node head 200(a) includes a transmitter 304(a) and a receiver 306(a),thereby providing two-way communications. However, in alternateembodiments, the node head 200(a) has only the transmitter 304(a) or thereceiver 306(a), thereby providing one-way communication. In anotherembodiment, the transmitter 304(a) and the receiver 306(a) are combinedinto a transceiver 308(a). Additionally, it is possible that node head200(a) include more than one transceiver, or an additional receiver ortransmitter to provide additional capabilities. Node head 200(b)includes a transmitter 304(b) and a receiver 306(b), thereby providingtwo-way communications. In one embodiment, the transmitter 304(b) andthe receiver 306(b) are combined into a transceiver 308(b).

Node base 202(a) includes a control module 310(a). Similarly, node base202(b) includes a control module 310(b). Each control module 310(a),310(b) receives signals from the receiver 306(a), 306(b) and controlsthe operation of its respective transmitter 304(a), 304(b) based on thereceived signal. More specifically, the control module 310(a), 310(b)interrupts or reestablishes the transmission of the transmitter 304(a),304(b). Thus, each control module 310(a), 310(b) controls its portion ofthe communication link with another node. The communication link isillustrated in FIG. 3 as including two communication beams 110(a),110(b).

The term “module,” as used herein, means, but is not limited to, asoftware or hardware component, such as a FPGA or ASIC, which performscertain tasks. A module may advantageously be configured to reside onthe addressable storage medium and configured to execute on one or moreprocessors. Thus, a module may include, by way of example, components,such as software components, object-oriented software components, classcomponents and task components, processes, functions, attributes,procedures, subroutines, segments of program code, drivers, firmware,microcode, circuitry, data, databases, data structures, tables, arrays,and variables. The functionality provided for in the components andmodules may be combined into fewer components and modules or furtherseparated into additional components and modules. Additionally, thecomponents and modules may advantageously be implemented to execute onone or more computers.

In operation, data that is transferred from node 108(a) to node 108(b)is modulated onto the communication beam 110(a) emitted by thetransmitter 304(a). Receiver 306(b) processes the received modulatedsignal in the communication beam 110(a) such that it can be repeated orforwarded to another node 108 in the network 100. Alternatively, theprocessed signal can be passed either to an end user at a facility 104or to a provider network 116 (see FIG. 1).

As mentioned above, the transmitter 304(b) can be interrupted due to anobject 312 being present in the optical communication beam 110(b). Theobject may be any opaque matter that sufficiently attenuates thetransmitted signal to a level such that the associated data is notdetectable by the receiver 306(a). In one embodiment, the object reducesthe power level of the communication beam 110(b) which is detected bythe receiver 306(a). For example, a bird, a baseball, smog, fog, or anairplane could block the beam of radiation. In one embodiment, the lowerbound signal-to-noise ratio that defines the block is selected based onthe error rate associated with the received data. In another embodiment,the block is defined based on the duration of the interruption.

FIG. 4 illustrates three different operating modes at different-timesthat may be implemented by the control module 310(a), 310(b) dependingon the status of the communication beams 110(a), 110(b). FIG. 4 depictsthe average power of a communication beam over time. Referring to FIGS.3 and 4, for example, when the communication beams 110(a), 110(b) arenot blocked and are properly targeted, the control modules 310(a),310(b) operate in a “normal operation” mode (Mode 1). In Mode 1, nodes108(a), 108(b) modulate data on their respective communication beams110(a), 110(b). The power levels of the communication beams 110(a),110(b) are set to a high level to achieve desired signal-to-noise ratiosat the respective receiver 306(a), 306(b), for example, 9.5 mW.

Assume, however, at a time T_(b), the object 312 blocks one or both ofthe communication beams 110(a), 110(b) between the nodes 108(a), 108(b).For example, in FIG. 3, communication beam 110(b) is blocked by object312. The power level of the communication beam 110(b) received by thereceiver 306(a) suddenly drops. The control module 310(a) responds tothis event by beginning the power reduction mode (Mode 2).

In the power reduction mode, the power level of the signal beingtransmitted by the transmitter 304(a) is immediately reduced to a lowlevel or zero after a short period T of delay. In one embodiment, periodT is 800 msec. The duration of T can be selected such that the totalenergy of the radiation transmitted by the transmitter 304(a) duringperiod T is below a level that would present a safety hazard to humans.For example, if the transmitter 304(a) was transmitting at an initialpower level of 9.5 mW during Mode 1, the maximum value of T is 0.85seconds. The control module 310(a) stops sending data on communicationbeam 110(a). Instead, the data received by node 108(a) that would havebeen sent to node 108(b) can be re-routed to an alternate node 108 (notshown) via one of the other node heads.

In response to the drop in power by node 108(a), the control module310(b) of node 108(b) can operate in a similar manner. Alternatively,the unblocked beam 110(a) can be left transmitting while a signal issent, via a network management system (not shown), to alert node 108(b)that beam 110(b) is not being received. When the second beam is forcedto fail, the control module 310(b) reduces the power of thecommunication beam 110(b) and stops sending data to node 108(a). Hence,blocking of a single communication beam 110(b) between two nodes 108(a),108(b) results in an interruption and failure of the two-waycommunication. However, this response may have a delay since the node108(b) is responding to the actions of node 108(a). By stopping thetransmission of the unblocked beam 110(a), an immediate signal, in theform of a lack of signal, is sent to the node transmitting the blockedbeam thus minimizing the complexity of notifying the blocked node andthe associated delay in such notification. The value of T is selected toaccount for this delay so that the radiation transmitted by thetransmitter 304(b) during T is also below a level that would present asafety hazard to humans.

Still referring to FIG. 4, once Mode 2 is executed and the output of thecommunication beam 110(b) is reduced to a safe level or shut off, thecontrol module 310(b) begins an acquisition and recovery mode (Mode 3).Mode 3 will continue until the communication beam 110(b) is no longerblocked. As shown in FIG. 4, in one embodiment the control module 310(b)operates the transmitter 304(b) in a pulsed transmission mode byintermittently raising its power to a high level for a short pulseduration, T_(d), with a time interval of T_(p). The power level duringeach pulse duration, T_(d), is sufficiently high so that thesignal-to-noise ratio at receiver 306(a) is acceptable for the purposeof reestablishing optical communication. In one embodiment, the powerlevel in each pulse is the same as the power level during the normaloperation mode (Mode 1). In another embodiment, the pulsed power levelis at a lower level. The communication beam 110(b) is modulated duringeach pulse duration, T_(d), with acquisition data for establishingoptical communication and is not modulated to carry data between pulses.The acquisition data may include, for example, a node ID, position, andorientation information. In another embodiment, the communication beam110(b) sends out other data along with the acquisition data during thepulse duration. In still another embodiment, the control module 310(b)alternates between the acquisition data and other data between eachpulse duration. The pulse duration T_(d) and the period T_(p) areselected so that the total radiation is below a level that would presentan unacceptable hazard to humans. Thus, during mode 3, the object 312 isnot exposed to a radiation level that would present a hazard to humans.

FIG. 5 is a graph of the power levels of an interrupted communicationbeam over time. FIG. 5 depicts an embodiment where Mode 3 includes atleast two different power levels, T_(d) ¹ and T_(d) ². Using differentpower levels can improve reestablishing optimal communication betweennodes 108(a), 108(b) even during adverse weather conditions. Forexample, on a clear day when visibility is good and the communicationbeam 110(b) is not blocked, the transmitter 304(b) operates at a highpower level, T_(d) ¹. However, such a high power level may saturatereceiver 306(a). To prevent this, the transmitter 304(b) transmits at alower power level during T_(d) ² so that the receiver 306(a) willproperly detect the communication beam 110(b) and be able to extract thetransmitted data. Conversely, the communication beam 110(b) transmittedat the low power level, T_(d) ², may be too weak on a foggy day toachieve a desired signal-to-noise ratio at the receiver 306(a). Bytransmitting at the high power level during T_(d) ¹, the receiver 306(a)will properly detect the communication beam 110(b) and be able toextract the transmitted data. Thus, this pulse structure allows twocommunicating nodes 108(a), 108(b) to reestablish optical communicationat local environmental and weather conditions throughout the year.

Still referring to FIG. 5, in one embodiment, the pulse durations T_(d)¹ and T_(d) ² are of equal duration and last for T_(d)/2. In anotherembodiment, both the high and low power levels, T_(d) ¹ and T_(d) ², aresufficiently high for communicating data to node 108(a). In stillanother embodiment, T_(d) ¹ and T_(d) ² are modulated to carry the samedata. In this embodiment, the data on the first half of the pulse, T_(d)¹, is at one power level (e.g., the high level) while the same data isreplicated on the second half of the pulse, T_(d) ², at a differentpower level (e.g., the low level). This dual-level pulse technique mayalso be used to accommodate communication links within the network 100architecture that have different node 108 distances. The pulse durationsT_(d) ¹ and T_(d) ² and the period T_(p) can be selected so that thetotal radiation exposure is below a level that would present anunacceptable hazard to humans.

The acquisition and recovery mode (Mode 3) is completed when both nodes108(a), 108(b) reestablish optical communication. In one embodiment,node 108(b) sends a “ping” to node 108(a) and expects an “echo” back. Ifnode 108(a) returns this “echo” through communication beam 110(a), node108(b) knows it has made a connection and that both communication beams310(a), 310(b) are not blocked. Alternatively, transmitter 304(a) sendsa “ping” to receiver 306(b). If receiver 306(b) receives the “ping,”control module 310(b) sends an “echo” through transmitter 304(b) back tonode 108(a).

At this point, the control modules 310(a), 310(b) of each node 108(a),108(b) terminate Mode 3 and begin the normal operating mode (Mode 1) asdiscussed above. As obvious to one skilled in the art, the controlsequence is not limited by the order of the modes discussed above. Forexample, the modes disclosed could be repeated in various orders withoutdisturbing the scope.

FIG. 6 is a block diagram of a control module 310(a) and/or 310(b)coupled to its associated transmitter 304 and receiver 306 from FIG. 3.The control module 310 includes a turret control module 600, a processor602, and a switch 604.

The transmitter 304 includes a power supply switch 914, a driver circuit916, and a laser 672. The power supply switch 914 drives power throughlaser 672. In one embodiment, the power switch 914 is a field effecttransistor (FET). The driver circuit 916 controls the output power anddata modulation of the laser 672 and can be independently controlled.Hence, in an event of blocking by an object, the output power of thelaser 672 is independently controlled from the power switch 914 and/orthe driver circuit 916.

The receiver 306 includes processing circuit elements 921 and an opticaldetector 704. The beam of a communication link that is transmitted bythe laser 672 is focused onto the optical detector 704. In oneembodiment, the optical detector 704 is a high-speed optical detectorsuch as, for example, a PIN photodiode detector or avalanche photodiodedetector (APD). The optical detector 704 is coupled to the processingcircuit elements 921. The processing circuit elements 921 generate twodifferent output signals 922 and 924 from the input signal received fromthe optical detector 704. The first signal 922 is the high-speed dataextracted from the received beam of radiation and sent to the switch604.

In one embodiment, the switch 604 is an ATM switch. ATM switches aregenerally well known in the art. Generally speaking, the ATM switchdetects an arriving cell, aligns boundaries of cells arriving onmultiple input lines, inspects the virtual path identifiers to determinethe routing for a cell, converts the serial stream into a word parallelformat, and time multiplexes the words onto time slots on a shared bus.A routing controller provides routing translation instructions torouting tables or accepts arriving virtual path identifiers from lineinterfaces to provide the correct routing instruction. A plurality ofrouting elements can be provided for each output. The routing elementinspects the routing instruction associated with each word appearing onthe shared bus, and delivers to its corresponding output cue only thosecell segments intended for that output. In the ATM embodiment, eachoutput cue reassembles the arriving word into ATM cells and deliverseach ATM cell to the corresponding output port in serial format.

The second signal 924 is a received signal strength indicator (RSSI)which indicates whether an incoming beam of radiation is blocked by anobject. The RSSI signal 924 is forwarded to the turret control module600. In one embodiment, the RSSI signal 924 is in analog form.

One embodiment of the turret control module 600 includes a programmablelogic device (PLD) 934, a digital multiplexer 931, a timer 933, and adigital pot 935. The PLD 934 provides local control intelligence for theturret control module 600 and includes a counter 936. The RSSI signal924 sent by the receiver 306 is received by the PLD 934 and an analog todigital (A/D) converter 942. When the RSSI signal 924 indicates ablocking has occurred at time T_(b) (see FIG. 4), the PLD 934 initiatesMode 2 operation after the delay time T to reduce or turn of the powerto the laser 672 in the transmitter 304. The delay time T in Mode 1, asillustrated in FIG. 4, is controlled by a timing signal from the timer933. Thus, once the RSSI signal 924 is lost, the counter 936 within thePLD 934 begins counting down the time. Once the counter 936 counts tothe end of the delay T, a signal 934 a is sent to turn off the laser 672or reduce its power via the driver circuit 916. The resulting powerlevel of the laser 672 is selected to limit the exposure of the objectto the beam of radiation. In one embodiment, the PLD 934 generates asecond signal 934 b that is coupled to the power switch 914 to turn offthe laser 672 or reduce its power, providing a single level ofredundancy.

Still referring to FIG. 6, the processor 602 includes the A/D converter942 which also receives the RSSI signal 924. The processor 602 runs orexecutes the modules described above and is programmed with software orfirmware (not shown) to perform the power control sequence illustratedin FIG. 4. The turret control module 600 interfaces with and receivescommands from the processor 602 via the digital multiplexer 931. Inresponse to commands from the processor 602, the digital multiplexer 931generates control signals 931 a, 931 b, 931 c. Signal 931 a is sent tothe PLD 934 to reset the counter 936. The signal 931 a is toggledperiodically, for example, every 500 msec or less, to continually resetthe counter 936 within the PLD 934. By continually resetting the counter936, the PLD signal 934 a is maintained at a value that keeps the laser672 at a desired power level during the acquisition and recovery mode(Mode 3). During Modes 1 and 2, the signal 931 a is not generated. Inone embodiment, the signal 931 a is left on during Modes 1 and 2 toallow continuous power to the laser 672.

The second control signal generated by the digital multiplexer 931 issignal 931 b. Signal 931 b controls both the PLD 934 and the powerswitch 914 in the transmitter 306. For example, if the processor 602receives the RSSI signal 924, via the A/D converter 942, and determinesthat the beam of radiation is blocked by an object, signal 931 b is setto a value that either turns off the power switch 914 or controls thepower switch 914 so that the power of the laser 672 is reduced to a safelevel. The signal 931 b is also fed to the PLD 934 instructing the PLD934 to set the value of the signal 934 a to turn off or reduce the powerof the laser 672 via the driver circuit 916. In another embodiment, thePLD 934 also sends signal 934 b to control the power switch 914. Besidesreceiving the RSSI signal 924, the processor 602 is also notified that ablock has occurred through a “loss of data” signal 951. The “loss ofdata” signal 951 is generated by the switch 604 when the high speed datasignal 922 is lost.

The third control signal generated by the digital multiplexer 931 issignal 931 c. Signal 931 c controls the digital pot 935. In response tosignal 931 c, the digital pot 935 controls the modulation power level ofthe driver circuit 916 of the transmitter 304.

Table A shows one example of the logic status of different signals inthe control module 310 for the control sequence described above.

TABLE A RSSI Control Signal Signal Signal Signal Signal Laser Mode 931a934a 931b 951 924 672 Laser is X Low Low X X Off commanded (Off) offAcquisition/ Run- High Mode 3 X X On Recovery ning (On) Wave- (Mode 3(Mode 3) form Wave- form) Normal Off High High Low High On (Mode 1) (On)(Data) Power Off Low Low High Low Off or at Reduction (Off) (no a safelow (Mode 2) data) power after delay T X = Do not care

Method of Operation

Operation of a communication network 100 in accordance with oneembodiment is described below with reference to FIGS. 7 and 8 along withreference to FIG. 3. For convenience of description, the following textdescribes the communication network 100 where a single communicationbeam 110(b) has been blocked by an object 312. However, the followingmethod can be used when both communication beams 110(a), 110(b) betweennodes 108(a), 108(b) are blocked.

The process begins at a start state 1000. Next, at a state 1002, anobject 312 blocks the communication beam 110(b). This may occur due toweather or an object, for example, a human or flying bird, entering thecommunication beam 110(b). Continuing to a state 1004, the controlmodule 310(a), through receiver 306(a), detects a power drop in thecommunication beam 110(b) from a transmitter 304(b). Next, at a state1006, in response to the drop in power, the control module 310(a) dropsthe power in a communication beam 110(a) sent by a transmitter 304(a)and stops sending data through transmitter 304(a) to node 108(b). Flowproceeds to state 1008 where the control module 310(a) re-routes thedata that was earmarked for receiver 306(b) through an alternate node(not shown). Next, at a state 1010, the control module 310(b), throughreceiver 306(b), detects a power drop in the communication beam 110(a)from transmitter 304(a). Flow continues to a state 1012 where, inresponse to the drop in power, the control module 310(b) drops thetransmission power of its communication beam 110(b) being sent by thetransmitter 304(b) to node 108(a). Next, at a state 1014, the controlmodule 310(b) stops sending data through transmitter 304(b) to receiver302(a). Flow moves to state 1016 where the control module 310(b)re-routes the data that was earmarked for receiver 306(a) through analternate node (not shown).

The acquisition and recovery process (Mode 3) performed by thefree-space optical communication system 100 will now be described withreference to FIG. 8. For convenience of description, the following textdescribes a free-space optical communication system 100 where a singlecommunication beam 110(b) is recovered. However, the acquisition andrecovery process can also be used when both communication beams 110(a),110(b) need to be recovered.

The free-space optical communication system 100 begins at a start state1100. Next, at a state 1102, a control module 310(b) transmits theacquisition information during T_(d) ¹ through transmitter 304(b). Flowproceeds to a decision state 1104 to determine if a receiver 306(a) ofnode 108(a) receives the transmission. In one embodiment, the controlmodule 310(b) sends a “ping” through transmitter 304(b) alongcommunication beam 110(b) and expects an “echo” back. If the “echo” isreceived by receiver 306(b) along communication beam 110(a), the controlmodule 310(b) knows it has made a connection. The free-space opticalcommunication system 100 then proceeds to an end state 1112 where theprocess terminates. Once Mode 3 terminates, Mode 1 is initiated.Referring again to decision state 1104, if the receiver 306(b) does notreceive the “echo” transmission, the free-space optical communicationsystem 100 continues to a state 1106 where transmitter 304(b) transmitsthe acquisition information during T_(d) ². Flow moves to decision state1108 to determine if the receiver receiving node received theinformation during T_(d) ². If the receiving node receives thetransmission, the free-space optical communication system 100 continuesto the end state 1112. Referring again to decision state 1108, ifreceiver 304(a) does not receive the transmission, the free-spaceoptical communication system 100 continues to a state 1110 where theacquisition and recovery process waits for the duration of T_(p)-T_(d)¹-T_(d) ². Flow then proceeds to state 1102 as described above to repeatthe transmissions.

FIG. 9 is a block diagram of an embodiment of a control module 310 fromFIG. 3 configured to optimize the characteristics of a receiver 306. Thecontrol module 310 depicted in FIG. 9 can be the control modules 310(a)and/or 310(b) shown in FIG. 3. In one embodiment, the receiver 306 canincorporate an Avalanche Photodiode Detector (APD) in its opticaldetector 704. In one embodiment, the APD operates at a rate at about 622Mb/s or higher. The control module 310 is coupled to its associatedtransmitter 304 and receiver 306 from FIG. 3. The control module 310includes a turret control module 1202, a processor 602, and a switch604.

The processor 602 runs or executes the modules described herein and isprogrammed with software or firmware (not shown) to perform theoptimization of the APD. The processor 602 is the same as previouslydescribed processor 602 (see FIG. 6) except for additional electricalconnections with the receiver 306 to allow the processor to optimize theoperation of the APD. These electrical connections provide the processor602, via A/D converter 942, with a temperature signal 1208 and a receive(Rx) power output signal 1210. Both signals, and their uses, will bedescribed below in detail.

The turret control module 1202 interfaces with and receives commandsfrom the processor 602 via digital multiplexer 931. The turret controlmodule 1202 is the same as previously described turret control module600 (see FIG. 6) except for the addition of a second digitalpotentiometer (pot) 1204. The second digital pot 1204 is electricallyconnected in parallel with previously described digital pot 935 (seeFIG. 6). In response to commands from the processor 602, the digitalmultiplexer 931 generates control signal 1214. Control signal 1214controls the digital pot 1204. In response to signal 1214, the digitalpot 1204 controls the modulation power level of the optical detector704, i.e. APD, via high voltage control signal 1206.

The processor 602, via the A/D converter 942, further provides anattenuation signal 1212 to the receiver 306. The receiver 306 uses theattenuation signal 1212 to activate an optical attenuator (not shown).The optical attenuator will be described in detail below with referenceto FIG. 15. The transmitter 304 and switch 604 are the same as describedabove with reference to FIG. 6.

FIG. 10 is a block diagram of a receiver 306 from FIG. 9 implementedwith an APD type optical detector 1300 and showing the componentsrelated thereto. The optical receiver 306 is configured to receive anon-off keyed (OOK) data modulated optical signal. The receiver 306 isfurther configured to convert the signal into a voltage level, and toamplify and retime the data with phase locked loop clock recovery. Theoperation of this example receiver circuit is now described. Afterreading this description, it will become apparent to one of ordinaryskill in the art how receiver 306 can be implemented with other receiverdetectors, architectures or configurations, or to receive signalsmodulated at wavelengths other than optical wavelengths.

In the embodiment illustrated in FIG. 10, light from a communicationlink, for example, 110(a) or 110(b) of FIG. 3, is focused onto anoptical detector 704. The optical detector 704 is a high-speed opticaldetector, such as, for example, an avalanche photodiode detector (APD)1300, to detect the total amount of power transmitted by a transmitter.The APD 1300 can include, for example, a 50-mm aperture 1302. Otherdetectors can be utilized to detect energy at optical or otherwavelengths depending on the application. Receiver 306 also includescomponents that provide control and temperature compensation of a highvoltage bias that is supplied to the APD 1300 for its operation.

A bias voltage is applied to the APD through series resistances locatedin a high voltage control (HVC) module 1304 and a current sense module1306. A power module 1310 provides power to a fixed high-voltage powersupply 1308. The fixed high-voltage power supply 1308 is further coupledto the HVC module 1304. The voltage operating range for the APD isdetermined during a calibration process which will be described belowwith reference to FIG. 12. During APD operation, the current sensemodule 1306 senses the bias voltage across the APD. The current sensemodule 1306 then amplifies the current that corresponds to the sensedvoltage. The output of the current sense module 1306 is then provided tothe processor 602 (see FIG. 9). This output is Rx power output signal1210 and functions as a power indication signal for the processor 602.

When light from the communication beam 110 is focused onto the activearea of the APD 1300, the APD 1300 generates a photo currentproportional to the intensity of the light. An amplifier 1311 convertsthe generated photo current to a voltage signal. In one embodiment, theamplifier is implemented as a high-speed transimpedance amplifier (TIA),which converts the photo current to a differential voltage signal. Anexample implementation of a high-speed TIA is the Maxim MAX3664transimpedance amplifier, available from Maxim Integrated Products, Incof Sunnyvale, Calif.

The voltage signal continues to a processing circuit 921. The processingcircuit includes a low pass filter 1312 configured to filter the voltagesignal to reduce high frequency noise prior to further amplification. Inone embodiment, low pass filter 1312 is a third order, 500 MHz, low passfilter, although other filters or band-pass frequencies can be used. Theprocessing circuit 921 further includes a data and retiming amplifier1314. The data and retiming amplifier 1314 provides furtheramplification of the voltage signal and re-times the data to a phaselocked loop internal clock. In one embodiment, the data and re-timingamplifier 1314 is implemented using a Maxim MAX3675 device providing ACcoupled differential emitter-coupled logic (ECL) outputs, re-timed to aphase-locked loop internal clock at a nominal data rate of 622Mbit/second. Other amplifiers can be implemented and can includealternative output levels and operate at alternative clock and datarates.

The output of the data and re-timing amplifier 1314 provides a receivedsignal strength indicator (RSSI) 924 and a high speed data signal 922.The RSSI signal 924 is sent to a programmable logic device (PLD) 934(see FIG. 9) and A/D converter 942 (see FIG. 9) for diagnostic purposes.The RSSI can be used to determine if the received signal is within thedynamic range of the receiver, whether the effective transmit powershould be adjusted, or for optical alignment purposes. The high-speeddata signal 922 is provided to high data rate switch 604 (see FIG. 9).

Receiver 306 further includes an APD temperature monitor 1316. The APDtemperature monitor 1316 generates a signal in the form of temperaturesignal 1208 indicating the temperature of the APD 1300. This is usefulwhere the operation of the receiver is highly temperature dependent. Forexample, the operating voltage of an exemplary APD 1300 changes at arate of 0.4 volts/° C. Temperature signal 1208 is used for diagnosticpurposes. As illustrated in FIGS. 9 and 10, the RSSI signal 924, Rxpower output 1210, and temperature signal 1208 are all provided to theprocessor 602 for diagnostic and control purposes.

The present techniques and devices include a detector circuit thatautomatically measures properties of the APD 1300 and accordinglyadjusts the electrical bias to the APD 1300 to improve its performance.Hence, different APDs coupled to such detector circuits may be biaseddifferently due to variations in the characteristics of the APD 1300.These techniques and devices allow each APD 1300 to be optimizedindividually with respect to the characteristics of that particular APD1300.

FIG. 11 is a schematic diagram of a current sense module from FIG. 10for use during a calibration process of the APD. Properties of the APDthat are measured during the calibration process may include, forexample, the maximum breakdown voltage, and the variation performancedue to changes in temperature of the APD. To this end, the current sensemodule 1306 is configured to obtain conduction measurements of the APD1300 during a calibration phase. The conduction measurements can be inthe form of a current or voltage. These measurements, along withtemperature signal 1208, are used by the processor 602 (see FIG. 9) todetermine a calibration value for the APD 1300. The current sense moduleincludes a resistor R_(A) 1400 and a differential amplifier 1402. Theresistor R_(A) 1400 is configured to measure the conduction across theAPD during the calibration process. During this calibration process, thevoltage received from the HVC module 1304 is incrementally increaseduntil conduction through the APD is measured by resistor R_(A) 1400. Thedifferential amplifier 1402 then amplifies the measured conduction. Theoutput of the differential amplifier 1402 is provided to the processor602 in the form of the Rx power output signal 1210. The processor 602uses the Rx power output signal to control the HVC module 1304 via theturret control module 1202.

FIG. 12 is a flow chart illustrating a calibration process for the APDthat is performed by the control module. The current sense module 1306senses the conduction, i.e. electric current, to the APD 1300 andproduces a feedback signal; i.e. Rx power output 1210, to adjust theactual bias to the APD 1300. Such adjustment is performed in thecalibration stage in which the breakdown bias voltage of a particularAPD 1300 is measured first and then the proper bias voltage foroperating that APD is set by reducing the breakdown bias by a desiredamount.

The calibration process begins at a start state 1500. Next, at a state1502, the transmit power to laser 672 (see FIG. 9) is turned off. Thisprevents any reflected light from reaching the APD 1300. Flow continuesto state 1504 where the HVC voltage is set to zero volts. Next, at astate 1506, the conduction of the APD is measured by the current sensemodule 1306 and is provided to the processor 602 via Rx power signal1210. Flow proceeds to a state 1508 where the processor 602 stores themeasured value in a memory (not shown). Next, at a state 1510, theprocessor 602 instructs the HVC module 1304 to incrementally increasethe voltage to the APD. More specifically, the processor 602 sendscommands to the digital multiplexer 931. In response to the commandsfrom the processor 602, the digital multiplexer 931 generates controlsignal 1214. Control signal 1214 controls the digital pot 1204. Inresponse to signal 1214, the digital pot 1204 controls the modulationoutput power level of the optical detector 704, i.e. APD, via highvoltage control signal 1206. Flow proceeds to a decision state 1512 todetermine if the breakdown current of the APD is exceeded. The breakdowncurrent corresponds to the maximum bias operating range for the APD1300. This breakdown current is determined by having the current sensemodule 1306 re-measure the APD conduction. If the current sense module1306 does not measure conduction through the APD, flow returns to state1510 where the processor 602 instructs the HVC module 1304 toincrementally increase the voltage to the APD. Flow then continues todecision state 1512 where the conduction through the APD is re-measured.

If the breakdown current of the APD is exceeded at decision state 1512,flow proceeds to state 1514 where the processor 602 subtracts a setpredetermined amount from the current value of the HVC module 1304.Next, at a state 1516, APD temperature module 1316 measures thetemperature of the APD 1300. This value is provided to the processor 602via temperature signal 1208. Flow proceeds to state 1518 where theprocessor 602 stores the proper calibrated value determined at state1514 and the temperature value measured at state 1516 in memory (notshown). Flow continues to a state 1520 where the processor 602, via theturret control board, sets the HVC module 1304 to the calibrated value.This calibrated value is the maximum voltage bias of the APD. Signaldrift due to the thermal effects can also be corrected by the feedbackto the HVC module 1304. The calibration process then proceeds to an endstate 1522 where the process terminates.

FIG. 13 is a schematic diagram of a high voltage control (HVC) module1304 from FIG. 10 which operates in conjunction with a resistor from thecurrent sense module to enhance the operational dynamic range of theAPD. The HVC module 1304 includes a resistor R_(B) 1600 located inseries between high-voltage supply 1308 and current sense module 1306.In one embodiment, the resistor R_(B) 1600 has a resistance of 100 Kohms. The HVC module 1304 further includes a feedback circuit forbiasing the output voltage to the current sense module 1306. In oneembodiment, the feedback circuit includes a field effect transistor(FET) 1602 located in series with a resistor R_(C) 1604, a resistorR_(D) 1606, and a differential amplifier 1608. In one embodiment,resistor R_(C) has a resistance of 182 K ohms and resistor R_(D) has aresistance of 10.2 M ohms. The differential amplifier 1608 receives highvoltage control signal 1206 from processor 602 via turret control board1202. The feedback circuit further includes capacitor 1610 and resistorR_(E) 1612. In one embodiment, capacitor 1610 has a value of 390 pF andresistor R_(E) has a resistance of 100 k ohms.

FIG. 14 is a graph of APD gain versus APD voltage bias, showing how thecurrent sense and HVC modules reduce both the APD gain and the APDvoltage bias in response to an increasing photo current whereby theAPD's operational dynamic range is increased. Along the x-axis is ameasure of the voltage bias of the APD over the APD's operating range.Along the y-axis is a measure of the gain of the APD over the APD'soperating range. Curve 1704 illustrates that when the photo current intothe APD increases, the voltage bias of the APD also decreases. At thesame time, the gain of the APD also decreases. This allows the APD tooperate over a wider dynamic range.

FIG. 15 is a block diagram of the transceivers 308(a), 308(b) from FIG.3, showing a reflected signal 1800 from receiver 306(b) interfering withcommunication beam 110(b) at receiver 306(a). When optimal weatherconditions occur between transceiver 308(a) and transceiver 308(b),transmitter 304(a) can oversaturate receiver 306(b). Laser power driftby transceiver 304(a) can also lead to saturation of receiver 306(b).Due to the limited dynamic range of the transmitter 304(a), its controlmodule may be unable to reduce the transmitter's power to preventoversaturation. Additionally, some of the incident light fromcommunication beam 110(a) creates a reflected signal 1800. The reflectedsignal 1800 is then reflected by the receiver 306(b) in a directiontowards the transceiver 308(a). Receiver 306(b), which is associatedwith transceiver 308(a), may experience interference with its incomingcommunication beam 110(b) from reflected signal 1800.

To reduce the incident light, an attenuator 1802(a) is located in thepath of communication beam 110(a). The attenuator 1802(a) can be anelectrochromatic window that is inserted in front of APD 1300. In oneembodiment, the attenuator 1802(a) is a light valve LCD iris. Uponapplication of a voltage to the attenuator 1802(a), its lighttransmission properties will change due the electrochromatic windowgoing from a bleached state to a colored state. The amount of lighttransmitted through the attenuator is lower when the attenuator is inthe colored state. In another embodiment, the attenuator is a photogreytype material. The photogrey type material changes its transmissionproperties upon application of a sufficient incident energy without theapplication of a voltage.

As illustrated in FIG. 15, the signal strength of communication beam110(a) is initially reduced at a point 1804 after passing throughattenuator 1802(a). The communication beam 110(a) is then reflected offof APD 1300 to form reflected signal 1800. However, reflected signal1800 is further reduced as it passes back through attenuator 1802(a). Inone embodiment, receiver 306(a) further includes attenuator 1802(b).Attenuator 1802(b) further reduces the signal strength of reflectedsignal 1800 before it interferes with incoming communication beam110(b).

Returning to FIGS. 9 and 10, an attenuator is shown located between APD1300 and communication beam 110(a). The attenuator is in electricalcommunication with attenuation control 1318. Attenuation control 1318controls the application of voltage to the attenuator. The attenuationcontrol receives an attenuation signal 1212 from processor 602 via A/Dconverter 942. As mentioned above, the processor 602 monitors thestrength of communication beam 110(a) via Rx power output 1210.Monitoring the strength of the communication beam 110(a) allows theprocessor 602 to dynamically activate and deactivate the attenuator viaattenuation control 1318. Attenuation control 1318 is further inelectrical communication with power supply 1310. The above describedconfiguration permits a control module 310 to monitor its own receivepower and independently control its own receiver 306 to stay within thereceiver's dynamic range.

FIG. 16 is a graph showing the transmission percent 1900 versuswavelength 1902 for the electrochromatic window. One line representswhen the electrochromatic window is activated and the other linerepresents when the electrochromatic window is deactivated. The amountof light that passes through the attenuator depends on the wavelength ofthe communication beam 110(a). For example, if the communication beam110(a) is transmitted at a wavelength of 785 nm into theelectrochromatic window, the signal strength of the communication beam110(a) is reduced approximately 20 db as shown by line 1904. If thereceiver 306(b) were near its saturation point for incident light, the20 db reduction would increase the effective saturation point uponactivation of the attenuator 1802. When the attenuator 1802 isdeactivated and in its bleached state, some minimum reduction in thesignal strength of the communication beam 110(a) also occurs. Forexample, in one embodiment this minimum reduction when the attenuator isin a bleached state is approximately 1.5 dB as shown by line 1906. Underpoor weather conditions, the deactivation of the attenuator 1802increases the energy level of the incident light to the APD 1300. Thus,the attenuator is activated and deactivated to keep the energy level ofthe incident light within the operational range of the APD 1300. Thisactivation and deactivation can occur in response to, for example,distance between transceivers 308(a), 308(b) (see FIG. 15), changingweather conditions, and laser power drift.

FIG. 17 is a graph of APD operating range versus time, showing theeffect of activating the electrochromatic window to reduce the photocurrent to the APD from a communication beam and thereby increase theAPD's operating range. As the photo current increases and nears themaximum operating range (i.e., saturation point) of the APD, theattenuator is activated at a time T. The activation of the attenuatorreduces the photo current into die APD and increases the APD's margin toits maximum operating range. As the photo current into the APD continuesto increase, represented by line 2008, the APD stays within itsoperating range.

FIG. 18 is a graph of APD operating range versus time, showing theeffect of deactivating the electrochromatic window to increase the photocurrent to the APD from a communication beam and thereby increase theAPD's operating range. As the photo current decreases and nears theminimum operating range of the APD along line 2104, the attenuator isdeactivated at a time T₁. The deactivation of the attenuator increasesthe photo current into the APD, represented by line 2106, and increasesthe APD's margin to its minimum operating range. As the photo currentinto the APD continues to decrease, represented by line 2108, the APDstays within its operating range.

FIG. 19 is a flow chart illustrating an attenuation process foradjusting the power of the incoming communication beam into the APD thatis performed by the control module. The attenuation process begins at astart state 2200. Next, at a state 2202, processor 602 monitors the Rxpower output signal 1210 of APD 1300 (see FIG. 9). Flow continues to adecision state 2204 where the processor 602 determines if the Rx poweroutput exceeds the saturation threshold level of the APD 1300. If the Rxpower output does not exceed the saturation threshold level of the APD1300, the process moves to a decision state 2206 where the processor 602determines if the Rx output power is below the minimum operatingthreshold level of the APD 1300. If the Rx power output level is abovethe minimum operating threshold of the APD 1300, the process returns tostate 2202 as described above where the processor 602 monitors the laserRx power and proceeds as described above.

Returning to decision state 2204, if the processor 602 determines thatthe Rx power output exceeds the maximum operating threshold level of theAPD 1300, the processes continues to a state 2216 where the attenuatoris enabled. Flow returns to state 2202 where processor 602 continues tomonitor the laser Rx power output as described above.

Returning to decision state 2206, if the processor 602 determines thatthe Rx power output is below the minimum operating threshold level ofthe APD 1300, the processes continues to a state 2208 where theattenuator is disabled. Flow returns to state 2202 where the processor602 continues to monitor the laser Rx power output as described above.

FIG. 20 is a block diagram of a receiver 306(a) from FIG. 3 whichincludes an electrochromatic window attenuator 2306 configured to beremoved from the path of communication beam 110(b). Attenuator 2306 issimilar to attenuator 1802(a) (see FIG. 15) except for being rotativelycoupled to the receiver 306(a) by pin 2300. Rotation of attenuator 2306along direction 2302 moves the attenuator out of the path ofcommunication beam 110(b). Rotation of attenuator 2306 along direction2304 moves the attenuator into the path of communication beam 110(b).Once rotated into the path of the communication beam 110(b), theattenuator 2306 is activated and/or deactivated as described above withreference to FIG. 19. Attenuation control 1318 (see FIG. 10) is furtherconfigured to control the rotation of the attenuator 2306 as prescribedby processor 602. Alternative positioning methods for moving theattenuator 2306 into and out of the path of the communication beam110(b) are within the scope of the disclosure.

FIG. 21( a) is a plan view of the attenuator window from FIG. 20,further configured to incrementally attenuate a communication beam110(b). Attenuator 2400 is similar to attenuator 2306 (see FIG. 20)except that the window includes a plurality of sectors 2402(a)-(p). Eachsector is configured for independent control by attenuation control 1318(see FIG. 10). For example, upon activation of sector 2402(a) byattenuation control 1318, sector 2402(a) transitions from a bleachedstate to a colored state. This activation reduces the photo current tothe APD by an incremental amount as compared to activating the entireattenuating window 2400. Activation of sector 2402(b) in addition toalready activated sector 2402(a) would further attenuate communicationbeam 110(b). Conversely, deactivation of sector 2402(b) would increasethe photo current to the APD. In an alternate embodiment, groups ofsectors are independently controlled by attenuation control 1318. Forexample, sectors 2402(a), 2402(b) are simultaneously activated ordeactivated by attenuation control 1318. In another embodiment, singlesectors 2402 are independently activated to reduce the photo current tothe APD while groups of sectors 2402 are deactivated to increase thephoto current to the APD. In still another embodiment, single sectors2402 are independently deactivated to increase the photo current to theAPD while groups of sectors 2402 are activated to decrease the photocurrent to the APD. An embodiment with entirely opaque, as with an LCD,segments is within the scope of the disclosure.

As shown in FIG. 21( b), non-adjacent sectors 2402(b), 2402(d), 2402(e),2402(g), 2402(j), 2402(l), 2402(m), 2402(o) are in a colored state. Inone embodiment, adjacent sectors are activated by attenuation control1318. The attenuation level for each incremental activation ordeactivation of a sector is determined from a ratio of the surface areacorresponding to the colored sectors to the area of the entireelectrochromatic window. For example, the attenuation level of anincoming communication beam for the attenuating window configurationshown in FIG. 21( b) is 50% of the maximum attenuation level.

FIG. 21( c) is a plan view of the attenuating window from FIG. 20,further configured to incrementally attenuate a communication beam110(b). Attenuator 2401 is similar to attenuator 2306 (see FIG. 20)except that the attenuating window includes a plurality of concentriccircles 2404(a)-(n). Each area between adjacent circles is configuredfor independent control by attenuation control 1318 (see FIG. 10). Forexample, upon activation of an area between circle 2404(a) and theperimeter of the attenuating window by attenuation control 1318, thearea transitions from a bleached state to a colored state. Thisactivation reduces the photo current to the APD by an incremental amountas compared to activating the entire attenuating window. Activation ofthe area between circle 2404(b) and circle 2404(a) in addition to thealready activated area between circle 2404(a) and the perimeter of theattenuating window would further attenuate communication beam 110(b).Deactivation of the area between circle 2404(a) and circle 2404(b) wouldincrease the photo current to the APD. In an alternate embodiment,groups of areas are independently controlled by attenuation control1318. For example, the area between circles 2404(b) and the perimeter ofthe attenuating window is simultaneously activated or deactivated byattenuation control 1318. In another embodiment, each area betweenadjacent circles 2404(a)-(n) are independently activated to reduce thephoto current to the APD while groups of areas are deactivated toincrease the photo current to the APD. In still another embodiment, eacharea between adjacent circles is independently deactivated to increasethe photo current to the APD while groups of areas are activated todecrease the photo current to the APD.

As shown in FIG. 21( d), the area formed by circles 2404(a) and 2404(b)along with a non-adjacent area formed by circle 2404(c) are both in acolored state. The adjacent areas are activated by attenuation control1318. The attenuation level for each incremental activation ordeactivation of an area(s) is determined from a ratio of the surfacearea corresponding to the colored area to the area of the entireattenuating window.

FIG. 22 is a flow chart illustrating an incremental attenuation processfor adjusting the power of the incoming communication beam into the APD1300 that is performed by the control module. The process shown includesthe engagement/disengagement capability for the attenuator but does notpreclude the possibility of an embodiment for incremental attenuationwithout the engagement/disengagement capability. The attenuation processbegins at a start state 2500. Next, at a state 2502, processor 602monitors the Rx power output signal 1210 of APD 1300. Flow continues toa decision state 2504 to determine if Rx power output is within theminimum and maximum threshold bounds. If the power is within the rangethe process continues to monitor the Rx power output as described above.

Returning to decision state 2504, if the Rx power output level exceedsthe maximum threshold level then the process continues to decision state2506 where the engagement of the attenuator is checked. If theattenuator is not engaged, i.e., in position, then the process continuesto state 2510 at which point the attenuator is engaged. The processcontinues to monitor the Rx power output as described above.

Returning to decision state 2506, if the attenuator is engaged then theprocess continues to state 2514 where the attenuation is incremented.The process continues to monitor the Rx power output as described above.

Returning to decision state 2504, if the Rx power level is below theminimum threshold level then the process continues to decision state2508 where the current attenuation level is checked. If the attenuatoris at the minimum then the process continues to state 2512 at whichpoint the attenuator is disengaged. The process continues to monitor theRx power output as described above.

Returning to decision state 2508, if the attenuation level is not at theminimum level then the process continues to state 2516 where theattenuation is decremented. The process continues to monitor the Rxpower output as described above.

FIG. 23 is a block diagram of a receiver from FIG. 9 showing amisaligned, incoming, communication beam 110 into an APD 1300.Communication beam 110 is misaligned an amount equal to angle 2600. Inone embodiment of optical transceivers 308(a), 308(b) (see FIG. 3), eachpair of an optical transmitter and an optical detector is mounted to amotorized turret and is fixed relative to each other. Hence, both thedirection of the optical transmitter and the direction of the opticaldetector change in the same manner with the movement of the turret. Anacquisition process in which two suitable turrets respectively locatedin two different nodes are pointed to each other and are alignedestablishes the communication link between the nodes. After the two-waycommunication is established, information can be transferred between thetwo nodes. To perform the alignment process, receiver 306 (see FIGS. 10and 23) is configured to generate two separate signals to represent thepower of a received communication beam 110 from another node. The firstsignal is a receive (Rx) power output signal 1210 of the APD, which canrepresent the full input power of the received communication beam.However, the Rx power output signal 1210 has low signal sensitivity whenthe input power to the APD is low. The second signal is a receivedsignal strength indicator (RSSI) 924 (see FIG. 10) which can besaturated at a relatively low power but can be used to measure the inputpower at low power levels.

FIG. 24 is a graph of Rx power output 2702 versus incident angle 2600for an incoming communication beam showing low sensitivity of the Rxpower output signal occurring at lower power levels. Line 2704 shows thesensitivity of the Rx power output at high power levels. At these highpower levels, processor 602 is able to align APD 1300 to the incomingcommunication beam by measuring the change in the Rx power output. Theprocessor rotates and/or elevates the receiver via actuators (not shown)while it measures the Rx power output. The receiver 306 is properlyaligned with the communication beam 110 when the measured Rx poweroutput is at its maximum value for any given incoming power level. Forexample, as the receiver rotates in directions 2708, 2710, the Rx powerlevel measured by processor 602 will increase or decreases. If the Rxpower level decreases, the receiver stops the rotation in that directionand begins rotating in the opposite direction. This continues untilrotation in either direction 2708, 2710 results in a reduction in the Rxpower output level measured by the processor. Once this occurs, thereceiver is properly aligned. However, at low power levels, Rx poweroutput sensitivity to changes in the incident angle is low. As shown byline 2706, when the sensitivity is low, the processor 602 is unable toproperly align the receiver with the communication beam 110.

FIG. 25 is a graph of receive signal strength indicator (RSSI) 2802versus incident angle 2600 for an incoming communication beam 110showing clipping of the RSSI occurring at higher power levels. Line 2804shows the sensitivity of the RSSI at low power levels. At these lowpower levels, processor 602 is able to align APD 1300 to the incomingcommunication beam by measuring the change in the RSSI. The processorrotates the receiver via actuators (not shown) while it measures theRSSI. The receiver 306 is properly aligned with the communication beam110 when the measured RSSI is at its maximum value for any givenincoming power level. For example, as the receiver rotates in directions2708, 2710, the RSSI measured by processor 602 will increase ordecreases. If the RSSI decreases, the receiver stops the rotation inthat direction and begins rotating in the opposite direction. Thiscontinues until rotation in either direction 2708, 2710 results in areduction in the RSSI measured by the processor. Once this occurs, thereceiver is properly aligned. However, at high power levels, RSSIsensitivity to changes in the incident angle is low. As shown by line2806, when the sensitivity is low, the processor 602 is unable toproperly align the receiver with the communication beam 110.

FIG. 26 is a graph of the RSSI and the Rx power output signal combined2902 versus incident angle 2600 for use with aligning an incomingcommunication beam 110 into the APD irrespective of power level. Powerlevel of the received beam may vary due to any single or combination offactors including, but not limited to, atmosphere attenuation, linkdistance resulting in geometric beam spread, and output laser power. Thecombined signal is used to indicate the received power level of the APDat both low and high power levels. For example, as the receiver rotatesin directions 2708, 2710, the combined signal measured by processor 602will increase or decreases. If the combined signal decreases, thereceiver stops the rotation in that direction and begins rotating in theopposite direction. This continues until rotation in either direction2708, 2710 results in a reduction in the combined signal level measuredby the processor. Once this occurs, the receiver is properly aligned.

Equations for deriving the combined signal based on Rx power output andRSSI are shown below. Equation P_(RSSI)(x) is the scaled curve-fit ofthe RSSI signal at low and high power.

${P_{RSSI}(x)}:={{{60000\mspace{14mu} {{if}\left\lbrack {{118000 \cdot {\exp \left\lbrack \frac{- \left( {x - 9964} \right)^{2}}{2 \cdot 16^{2}} \right\rbrack}} > 60000} \right\rbrack}}}{118000 \cdot {\exp \left\lbrack \frac{- \left( {x - 9964} \right)^{2}}{2 \cdot 16^{2}} \right\rbrack}}\mspace{14mu} {otherwise}}$x = 9920  …  10000 P_(RSSI)(9980) = 6 ⋅ 10⁴

Equation P_(Rx)(x) is the scaled curve-fit of the Rx power signal at lowand high power.

${P_{Rx}(x)} = {1000 \cdot {\exp \left\lbrack \frac{- \left( {x - 9965} \right)^{2}}{2 \cdot 10^{2}} \right\rbrack}}$

Equation P_(combined)(x) is the scaled sum of equation P_(RSSI)(x) andequation P_(Rx)(x).

P _(combined)(x)=1024·P _(Rx)(x)+P _(RSSI)(x)

These equations are derived empirically from the characterization of theresponse of the RSSI and Rx power circuitry. Each is a fitted Guassianrepresenting the communication beam profile with appropriate derivedscaling factors to yield a composite signal. This composite signal isthus valid over a large range of power.

While the above detailed description has shown, described, and pointedout novel features as applied to various embodiments, it will beunderstood that various omissions, substitutions, and changes in theform and details of the device or process illustrated may be made bythose skilled in the art without departing from the spirit. The scope isindicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

1. A method comprising: receiving a communication beam by an opticaldetector; monitoring a Receive (Rx) power level indicative of strengthof the communication beam received by the optical detector; determiningif the Rx power level exceeds a saturation threshold level for theoptical detector; if the Rx power level exceeds a saturation thresholdlevel of the optical detector, incrementing attenuation of thecommunication beam by incrementally activating an optical attenuator,wherein the optical attenuator is located in a path of the communicationbeam and the optical detector, wherein the optical attenuator isoperable to attenuate the strength of the communication beam received bythe optical detector; wherein the optical attenuator has a first area,wherein said incrementally activating the optical attenuator comprisesturning on a first portion of the optical attenuator, wherein the firstportion is smaller than the first area.
 2. The method of claim 1,further comprising: if the optical detector is not engaged, engaging theoptical attenuator.
 3. The method of claim 2, wherein the opticalattenuator is movable into and out of the path of the communicationbeam.
 4. The method of claim 1, wherein a communication system comprisesa first node and a second node, wherein the second node is configured totransmit the communication beam to the first node, and wherein the firstnode includes the optical attenuator.
 5. The method of claim 1, whereinthe first area comprises a plurality of sectors; and wherein the firstportion comprises one or more sectors of the plurality of sectors. 6.The method of claim 1, wherein the first area comprises a plurality ofconcentric circles; and wherein the first portion comprises one or moreconcentric circles of the plurality of concentric circles.
 7. The methodof claim 1, wherein the optical detector comprises one or more of: anavalanche photodiode detector (APD); a charge coupled device (CCD); aphotomultiplier tube (PMT); or any other type of a photo detector. 8.The method of claim 1, further comprising: converting the communicationbeam into a first signal; measuring the strength of the first signal toproduce a received signal strength indicator (RSSI) for the firstsignal; and aligning the optical detector with the communication beambased on one or more of the RSSI or the Rx power level.
 9. A methodcomprising: receiving a communication beam by an optical detector;monitoring a Receive (Rx) power level indicative of strength of thecommunication beam received by the optical detector; determining if theRx power level is below a minimum threshold level for the opticaldetector; if the Rx power level is below the minimum threshold level ofthe optical detector, decrementing attenuation of the communication beamby incrementally deactivating an optical attenuator, wherein the opticalattenuator is located in a path of the communication beam and theoptical detector, wherein the optical attenuator is operable toattenuate the strength of the communication beam received by the opticaldetector; wherein the optical attenuator has a first area, wherein saidincrementally deactivating comprises turning off a first portion of theoptical attenuator, wherein the first portion is smaller than the firstarea.
 10. The method of claim 9, further comprising: if the attenuationof the optical attenuator is below a minimum level, disengaging theoptical attenuator.
 11. The method of claim 9, wherein the opticalattenuator is movable into and out of the path of the communicationbeam.
 12. The method of claim 9, wherein a communication systemcomprises a first node and a second node, wherein the second node isconfigured to transmit the communication beam to the first node, andwherein the first node includes the optical attenuator.
 13. The methodof claim 9, wherein the first area comprises a plurality of sectors; andwherein the first portion comprises one or more sectors of the pluralityof sectors.
 14. The method of claim 9, wherein the first area comprisesa plurality of concentric circles; and wherein the first portioncomprises one or more concentric circles of the plurality of concentriccircles.
 15. The method of claim 9, wherein the optical detectorcomprises one or more of: an avalanche photodiode detector (APD); acharge coupled device (CCD); a photomultiplier tube (PMT); or any othertype of a photo detector.
 16. The method of claim 9, further comprising:converting the communication beam into a first signal; measuring thestrength of the first signal to produce a received signal strengthindicator (RSSI) for the first signal; and aligning the optical detectorwith the communication beam based on one or more of the RSSI or the Rxpower level.
 17. A system comprising: an optical detector configured toreceive a communication beam, wherein the optical detector is operableto generate a receive (Rx) power level indicative of strength of thecommunication beam; an optical attenuator located in a path of thecommunication beam and the optical detector, wherein the opticalattenuator is operable to attenuate the strength of the communicationbeam received by the optical detector, wherein the optical attenuatorhas a first area; and an attenuator control module coupled to theoptical attenuator, wherein the attenuator control module is operable toreceive the Rx power level and control the optical attenuator based onthe Rx power level; wherein if the Rx power level exceeds a saturationthreshold level of the optical detector, the attenuator control moduleis operable to send one or more control signals to the opticalattenuator incrementing attenuation of the communication beam, whereinsaid incrementing attenuation comprises turning on a first portion ofthe optical attenuator, wherein the first portion is smaller than thefirst area.
 18. A system comprising: an optical detector configured toreceive a communication beam, wherein the optical detector is operableto generate a receive (Rx) power level indicative of strength of thecommunication beam; an optical attenuator located in a path of thecommunication beam and the optical detector, wherein the opticalattenuator is operable to attenuate the strength of the communicationbeam received by the optical detector, wherein the optical attenuatorhas a first area; and an attenuator control module coupled to theoptical attenuator, wherein the attenuator control module is operable toreceive the Rx power level and control the optical attenuator based onthe Rx power level; wherein if the Rx power level is below a minimumthreshold level of the optical detector, the attenuator control moduleis operable to send one or more control signals to the opticalattenuator decrementing attenuation of the communication beam, whereinsaid decrementing attenuation comprises turning off a first portion ofthe optical attenuator, wherein the first portion is smaller than thefirst area.